Serine 113 is the site of receptor-mediated phosphorylation of the Dictyostelium G protein α-subunit Gα2

The G protein α-subunit Gα2 is essential to the developmental program of Dictyostelium. Gα2 is transiently phosphorylated on a serine residue(s) following stimulation with extracellular cAMP (Gundersen, R. E., and Devreotes, P. N. (1990) Science 248, 591-593). To aid in defining the function of α-subunit phosphorylation, we identified the site of Gα2 phosphorylation. Comparison of the isoelectric points (pI) of the phosphorylated and nonphosphorylated forms indicated that a single mole of phosphate is added to Gα2. Cleavage at tryptophan residues and immunoprecipitation with a specific peptide antibody localized the phosphorylated serine in the N-terminal 119 residues. Analysis of a series of Gα1 and Gα2 chimeras further confined the site between amino acids 33 and 215. Site-directed mutagenesis of serines between amino acids 33 and 119 produced two mutants that were not phosphorylated, S45A and S113A. Ser¹¹³ was identified as the site by sequential Edman degradation of ³²P- radiolabeled Gα2 digested with endoproteinase Glu-C. We have expressed the Gα2 mutants S113A, S113I, S113T, and S113D in a Gα2 null cell line to examine the function of phosphorylation.